Dust discrimination for sensing systems

ABSTRACT

A sensing system and method for detecting particles in an air volume includes a main airflow path including an inlet from the air volume. Also included is a collection site within the main airflow path. The collection site is for drawing an air sample from the main airflow path. Additionally the system includes a means to induce a localized increase in particle speed at the collection site relative to air speed along the remainder of the main airflow path. The means to induce a localized increase includes an auxiliary airflow path from the main airflow path. The auxiliary airflow path has an exit upstream of the collection site. In an alternative form of the invention, the means to induce the localized increase in particle speed at the collection site comprises a venturi in the main airflow path with the collection site being disposed along the venturi.

FIELD OF THE INVENTION

The present invention relates to a sensing system and method fordetecting particles in an air volume. More particularly, although notexclusively, the invention relates to an aspirating smoke detector.However, the invention is not limited to this particular application andother types of sensing systems for detecting particles in an air volumeare included within the scope of the present invention.

BACKGROUND OF THE INVENTION

Smoke detection systems can be falsely triggered by exposure to dust. Inaspirating smoke detection systems, various analytical solutions havebeen implemented in order to detect the dust and thereby avoid a falsealarm. In light-scatter-based smoke detection systems, dustdiscrimination may be implemented by using time-amplitude analysis(individual dust particles or small clouds of dust tend to produce aspike in the scatter signal) or by using multiple light wavelengths,multiple polarisations, multiple viewing angles or combinations of theabove. These analytical tools add complexity to the smoke detectionsystems.

Another means to limit problems associated with dust is to limit theintroduction of dust into the smoke detection chamber. This is achievedthrough various means, such as filtration or inertial separation inwhich smaller particles suspended in an air stream are preferentiallydrawn into the sample site. These smaller particles are more likely tobe associated with smoke than dust. The momentum of larger and heavierparticles tend to take them past the sampling port volume without beingdrawn into it.

By way of example, FIG. 1 illustrates the operation of a typicalinertial separator. The air flow A travels within the sampling pipe D toa sampling port of sampling collection tube E. The heavier and largerparticles B are carried past the sampling port of the samplingcollection tube while a proportion of the smaller, lighter particles Care drawn into the sampling port and then to the sampling site foranalysis. The remainder of the airflow F leaves the system.

The problem with inertial separation is that it is not effective unlessthere is rapid air movement in the sampling pipe D. However, this rapidair movement within the sampling pipe D is not always practical,requiring a larger fan with higher energy requirements than wouldotherwise be required.

It is therefore an object of the present invention to provide animproved sensing system which addresses the abovementioned disadvantage,or at least provides the public with a useful choice over known systems.

Reference to any prior art in the specification is not, and should notbe taken as, an acknowledgment or any form of suggestion that this priorart forms part of the common general knowledge in Australia or any otherjurisdiction or that this prior art could reasonably be expected to beascertained, understood and regarded as relevant by a person skilled inthe art.

SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention, there is provideda sensing system for detecting particles in an air volume, the sensingsystem including:

-   -   a main airflow path including an inlet from the air volume;    -   a collection site within the main airflow path, the collection        site for drawing an air sample from the main airflow path; and    -   means to induce a localised increase in particle speed at the        collection site relative to air speed along the remainder of the        main airflow path, the means to induce a localised increase        including an auxiliary airflow path from the main airflow path,        the auxiliary airflow path having an exit upstream of the        collection site.

The means to induce a localised increase in the particle speed at thecollection site may include a means to increase air speed and/or airflowat the collection site. The auxiliary airflow path recirculates airdrawn from the main airflow path. Thus air in the main air flow path,parallel to the auxiliary air flow path, thereby flows at a higher speedthan the airflow along the remainder of the main airflow path. Theauxiliary airflow path may include a fan to draw air into the auxiliaryairflow path.

In a most preferred form of the invention, the auxiliary airflow pathmay be in the form of a circulating loop. Preferably, the entrance tothe circulating loop is downstream from the collection site and the exitfrom the circulating loop is upstream from and adjacent to thecollection site. In a most preferred form of the invention, the exit ofthe circulating loop projects into the main airflow path immediatelyadjacent to the collection site. The exit of the auxiliary airflow pathor circulating loop may be smaller than the opening so that the exitspeed of the air is greater than the speed at the opening (being drawnfrom the main airflow path).

The collection site suitably employs the principles of inertialseparation for promoting collection of smaller particles in favour oflarger particles. The collection site may comprise a sample collectiontube projecting, into the main airflow path. Preferably, the samplecollection tube has an outer surface that is closed at the portion whichfaces the upstream direction of airflow. The opening for collectingparticles from the airflow is formed in the outer surface portion of thecollection tube which faces downstream of the airflow.

The sensing system may include a detector to which the sample flows fortesting. The detector may be in the form of a particle detector such asa light-scatter particle detector. Preferably, the particle detector isa smoke detector. Suitably, the detector can be connected to a firealarm system and optionally, to an appropriate suppressant system.

The airflow path suitably includes a sampling pipe network including asampling pipe and inlet ports. The sensing system preferably includes asub-sampling path that draws air from the collection site and includes afan, a detection chamber and optionally, a filter.

In accordance with a second aspect of the present invention, there isprovided a method of sensing particles in an air volume, the methodcomprising:

-   -   drawing air from the air volume and causing airflow along a main        airflow path past a collection site;    -   inducing a localised increase in particle speed at the        collection site, relative to air speed along the remainder of        the main airflow path using an auxiliary airflow path from the        main airflow path, the auxiliary airflow path having an exit        upstream of the collection site; and    -   testing an air sample drawn from the collection site.

The step of inducing the localised increase in particle speed may benon-continuous and may depend on the result of a pre-testing step. Theauxiliary airflow path such as a circulating loop may include a fan tocreate the localised increase in particle speed at the collection site.This fan may run continuously so that all of the air samples drawn fromthe collection site have the benefit of improved inertial separation.Alternatively, the fan may run intermittently. In one preferred method,there may be an initial pre-testing step which operates periodically orintermittently. If the pre-testing step detects a predetermined level ofparticulate matter then the fan is operated to induce the localisedincrease in particle speed and testing of the air sample is thenconducted to take a reading of the air sample under the conditions morefavourable to particle mass discrimination.

In accordance with a third aspect of the present invention, there isprovided a sensing system for detecting particles in an air volume, thesensing system including:

-   -   a main airflow path which is substantially straight, the main        airflow path including an inlet from the air volume;    -   a collection site in the form of a sample tube projecting into        the main airflow path, the sample tube for drawing an air sample        from the main airflow path; and    -   means to induce a localised increase in particle speed at the        collection site relative to air speed along the remainder of the        main airflow path, the means to induce the localised increase in        particle speed at the collection site comprising a venturi in        the main airflow path with the collection site being disposed        along the venturi.

In accordance with a fourth aspect of the present invention, there isprovided a method of sensing particles in an air volume, the methodcomprising:

-   -   drawing air from the air volume and causing airflow along a        substantially straight main airflow path past a collection site;    -   inducing a localised increase in particle speed at the        collection site, relative to air speed along the remainder of        the main airflow path, by the use of a venturi in the main        airflow path, with the collection site being disposed along the        venturi;    -   inducing a transverse airflow along a transverse flow path which        is transverse to the main airflow path; and    -   testing the transverse airflow.

In accordance with a fifth aspect of the present invention, there isprovided a smoke sensing system for detecting particles in an airvolume, the sensing system including:

-   -   a main airflow path including an inlet from the air volume;    -   a collection site for drawing an air sample from the main        airflow path;    -   means to induce a localised increase in particle speed at the        collection site relative to air speed along the remainder of the        main airflow path, the means to induce the localised increase in        particle speed at the collection site comprising a venturi in        the main airflow path with the collection site being disposed        along the venturi; and    -   a smoke detector for testing the air sample.

In accordance with a sixth aspect of the present invention, there isprovided a method of sensing smoke particles in an air volume, themethod comprising:

drawing air from the air volume and causing airflow along a main airflowpath past a collection site;

-   -   inducing a localised increase in particle speed at the        collection site, relative to air speed along the remainder of        the main airflow path, by the use of a venturi in the main        airflow path with the collection site being disposed along the        venturi;    -   inducing a sample flow away from the main airflow path; and    -   testing the sample flow for smoke particles.

Preferably, in the third to sixth aspects recited above, the collectionsite is disposed at a narrow portion or the narrowest part of theventuri.

As used herein, except where the context requires otherwise, the term“comprise” and variations of the term, such as “comprising”, “comprises”and “comprised”, are not intended to exclude further additives,components, integers or steps.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

Exemplary embodiments of the present invention will now be described byway of non-limiting example only, with reference to the accompanyingdrawings, in which:

FIG. 1 illustrates a prior art arrangement of a sensing system with aninertial separator;

FIG. 2 illustrates a sensing system according to a preferred embodimentof the present invention;

FIG. 3 illustrates an alternative embodiment of the sensing system ofFIG. 2; and

FIG. 4 illustrates a further embodiment of the sensing system.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The sensing system of FIG. 2 includes a sampling pipe D which definesthe main airflow path A,F. The main airflow path A,F passes a samplecollection tube E projecting into the main airflow path A,F. Thesampling collection tube E has an opening which faces downstream of themain airflow path and is used to draw an air sample which then passed toa test chamber for analysis. A sub-sample of the main airflow is drawninto the sampling collection tube E and the momentum of the larger andheavier particles B tend to take them past the opening of the samplecollection tube E without being drawn into the opening. The smaller,lighter particles C are drawn into the sampling collection tube E fromwhere they are drawn away to the detector for analysis (not shown).

A circulating loop K has an entrance J downstream of the samplingcollection tube E and exits at I back into the main airflow path A,upstream of the sampling collection tube E. The circulating loop Kincorporates a fan G which draws airflow H through the circulating loopK. The recirculating air increases the speed of the airstream flowingthrough the main loop along M parallel to the circulating loop K, andpast the opening of the sampling collection tube E. However, thislocalised increased speed of airflow in the circulating loop K haslittle or no, effect on the flows upstream A and downstream F of therecirculating airstream H,M.

In FIG. 3, the use of similar or the same reference numerals as FIG. 2illustrates like parts, the prime symbol (′) indicating where a part hasbeen modified for the new embodiment.

FIG. 3 illustrates an enhancement whereby the exit I′ of the circulatingloop K′ projects into the main airflow path adjacent and upstream of thesampling collection tube E. The exit I′ has an orifice of much smallerdiameter than the downstream entrance J′ to the circulating loop K′.Therefore, the exit speed of the airstream L′ at the exit I′ is muchgreater than the speed of the inlet airflow H′.

The sensing system of FIGS. 2 and 3 can be operated in at least twoways. Firstly, the duct fan G may run continuously to assist inproviding ongoing separation between large and small particles. This hasthe advantage of ease of operation but has two drawbacks. The first isthe lifespan of the fan G is reduced due to its continuous operation andexposure to background dust and smoke levels. The second drawback isthat continuous operation of the fan G continuously consumes power.

Alternatively, the fan may be operated by only turning it on when someparticulate is detected by the sensing system. The change in readingonce the fan is operating is indicative of the relative amount of largeand small particles in the airstream and thus can be used to give anindication as to whether the detected particulate is dust or smoke, orprovide other indication of the nature of the particles or conditionbeing sensed. Because the fan G is operated infrequently, it is likelyto last longer. Another advantage of this arrangement is that when thefan is off it consumes no power.

In a third embodiment illustrated in FIG. 4, a venturi is employed inthe sampling pipe D with the sampling collection tube being disposed ator adjacent the throat of the venturi where the air speed increases.This embodiment does not increase the total airflow past the samplingcollection tube, but increases the local airspeed at its location,thereby increasing momentum of the particles entrained in the airflowand increasing the inertial separation effect of the sampling collectiontube. This embodiment has the advantage of not adding any moving partsto the system.

The foregoing describes only one embodiment of the present invention andmodifications may be made thereto without departing from the scope ofthe invention.

It will be understood that the invention disclosed and defined in thisspecification extends to all alternative combinations of two or more ofthe individual features mentioned or evident from the text or drawings.All of these different combinations constitute various alternativeaspects of the invention.

1. A sensing system for detecting particles in an air volume, the sensing system including: a main airflow path including an inlet from the air volume; a collection site within the main airflow path, the collection site for drawing an air sample from the main airflow path; and means to induce a localized increase in particle speed at the collection site relative to air speed along the remainder of the main airflow path, the means to induce a localized increase including an auxiliary airflow path from the main airflow path, the auxiliary airflow path having an exit upstream of the collection site.
 2. The sensing system as claimed in claim 1 wherein the auxiliary airflow path includes a fan to draw air into the auxiliary airflow path.
 3. The sensing system as claimed in claim 1 wherein the auxiliary airflow path is in the form of a circulating loop with an entrance to the circulating loop being downstream from the collection site and the exit from the circulating loop being upstream from the collection site.
 4. The sensing system as claimed in claim 3 wherein the exit of the circulating loop projects into the main airflow path immediately adjacent to the collection site.
 5. The sensing system as claimed in claim 1 wherein the exit of the auxiliary airflow path or circulating loop is smaller than its entrance so that the exit speed of the air is greater than the speed at the entrance.
 6. A method of sensing particles in an air volume, the method comprising: drawing air from the air volume and causing airflow along a main airflow path past a collection site; inducing a localized increase in particle speed at the collection site, relative to air speed along the remainder of the main airflow path using an auxiliary airflow path from the main airflow path, the auxiliary airflow path having an exit upstream of the collection site; and testing an air sample drawn from the collection site.
 7. The method as claimed in claim 6 wherein the auxiliary airflow path includes a fan to draw air into the auxiliary airflow path.
 8. The method as claimed in claim 7 wherein the fan is run continuously.
 9. The method as claimed in claim 7 wherein the fan is run intermittently.
 10. The method as claimed in claim 9, further including: pre-testing an air sample drawn from the collection site to determine the level of particulate matter in the sample; determining whether the level is above a predetermined threshold; running the fan; and testing another air sample drawn from the collection site.
 11. A sensing system for detecting particles in an air volume, the sensing system including: a main airflow path which is substantially straight, the main airflow path including an inlet from the air volume; a collection site in the form of a sample tube projecting into the main airflow path, the sample tube for drawing an air sample from the main airflow path; and means to induce a localized increase in particle speed at the collection site relative to air speed along the remainder of the main airflow path, the means to induce the localized increase in particle speed at the collection site comprising a venturi in the main airflow path, with the collection site being disposed along the venturi.
 12. The sensing system as claimed in claim 11 wherein the collection site is disposed at a narrow portion or the narrowest part of the venturi.
 13. A method of sensing particles in an air volume, the method comprising: drawing air from the air volume and causing airflow along a substantially straight main airflow path past a collection site; inducing a localized increase in particle speed at the collection site, relative to air speed along the remainder of the main airflow path, by the use of a venturi in the main airflow path, with the collection site being disposed along the venturi; inducing a transverse airflow along a transverse flow path which is transverse to the main airflow path; and testing the transverse airflow.
 14. The method as claimed in claim 13 wherein the collection site is disposed at a narrow portion or the narrowest part of the venturi.
 15. A smoke sensing system for detecting particles in an air volume, the sensing system including: a main airflow path including an inlet from the air volume; a collection site for drawing an air sample from the main airflow path; means to induce a localized increase in particle speed at the collection site relative to air speed along the remainder of the main airflow path, the means to induce the localized increase in particle speed at the collection site comprising a venturi in the main airflow path with the collection site being disposed along the venturi; and a smoke detector for testing the air sample.
 16. The sensing system as claimed in claim 15 wherein the collection site is disposed at a narrow portion or the narrowest part of the venturi.
 17. A method of sensing smoke particles in an air volume, the method comprising: drawing air from the air volume and causing airflow along a main airflow path past a collection site; inducing a localized increase in particle speed at the collection site, relative to air speed along the remainder of the main airflow path, by the use of a venturi in the main airflow path, with the collection site being disposed along the venturi; inducing a sample flow away from the main airflow path; and testing the sample flow for smoke particles.
 18. The method claimed in claim 17 wherein the collection site is disposed at a narrow portion or the narrowest part of the venturi. 